This study will examine the presynaptic effects of chemical mediators on the stimulation-evoked release of norepinephrine and acetylcholine in the isolated, perfused iris-ciliary body of rabbits. Specific agents to be tested include adrenergic agonists and antagonists, cholinergic drugs, and a variety of neuromodulators reported to inhibit or facilitate the release of neurotransmitter at autonomic synapses in other target tissues. The presynaptic regulatory effects of drugs used in glaucoma therapy will be evaluated in iris-ciliary bodies isolated from drug-treated rabbits. An important goal of this study is to develop an appropriate in vitro model for screening the potential presynaptic actions of drugs used in clinical opthalmology. Results from these experiments may suggest new approaches to treatment to treatment of glaucoma and disorders of the autonomic nervous system.